1. Field of the Invention
The present invention relates to a Coriolis mass flowmeter having im disposed on a straight line which contains said median point and proved stability, measuring accuracy, and vibration-proof performance.
2. Description of the Related Art
In order to facilitate understanding of the present invention, a description will be given of problems encountered with known arts, with reference to FIGS. 74 to 78.
FIG. 74 shows the structure of a conventional apparatus disclosed, for example, in Japanese Unexamined Patent Application Publication No. 6-109512.
Referring to this Figure, a vibration tube 1 has both ends connected to flanges 2 which are used for connecting the vibration tube 1 between conduits. An ocillator 3 is fixed to a median point of the vibration tube 1.
Vibration sensors 4 and 5 are positioned near both ends of the vibration tube 1. The vibration tube 1 is fixed at its both ends to a housing 6.
With this arrangement, a fluid as a measuring object is caused to flow through the vibration tube 1 while the ocillator 3 is activated.
A Coriolis force expressed by the following equation is generated, where the angular velocity of the vibration caused by the ocillator 3 is expressed by [xcfx89] and the flow velocity of the fluid is expressed by [V], with each symbol in brackets indicating a vector quantity. The mass flow rate of the fluid can be determined by measuring the vibration which is proportional to the Coriolis force.
Fc=xe2x88x922m[xcfx89]xc3x97[V]
FIG. 75 shows the structure of another conventional apparatus disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-108723.
A vibration tube 11 performs a simple harmonic oscillation or a circular motion on a circle which is at a predetermined radial distance from each point on a reference axis 14 defined as a straight line interconnecting an upstream fixed end 12 and a downstream fixed end 13 of the vibration tube 11.
An ocillator 15 is provided on the median point of the vibration tube 11. Vibration sensors 16 and 17 are disposed near both ends of the vibration tube 11.
FIG. 76 is a cross-sectional view of the vibration tube 11 taken along the line bxe2x80x94b of FIG. 75. FIG. 77 is a cross-sectional view of the vibration tube 11 taken along the line axe2x80x94a or cxe2x80x94c of FIG. 75. FIG. 78 is a perspective view illustrating the manner in which the vibration tube 11 vibrates.
Referring to FIGS. 76 and 77, the vibration tube 11 when not ocillated is held near a position indicated by xe2x80x9cAxe2x80x9d.
When the vibration tube 11 is ocillated, the center of the vibration tube 11 moves on a circle of a radius R(x) from the reference axis 14.
At the position of the cross-section bxe2x80x94b, the center of the vibration tube 11 oscillates on an arc or a part of a circle having a radius R(b) from the reference axis 14. At the position of the cross-section axe2x80x94a or cxe2x80x94c, the center of the vibration tube 11 oscillates on an arc or a part of a circle of a radius R(a) or R(c) from the reference axis 14, from a position A to a position B and from the position B to the position A, and from the position A to a position C and then again to the position A, and repeats this operation.
Symbols xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d in FIG. 78 respectively correspond to the positions of the vibration tube 11 indicated by the same symbols in FIGS. 76 and 77. Numerals 12 and 13 denote fixed ends of the vibration tube 11, and 14 denotes the reference axis which is the straight line interconnecting these fixed ends.
Since each point on the vibration tube 11 oscillates only on an arc or a part of circle which is at a constant distance from the reference axis 14, the length of the vibration tube 11 is held constant regardless of the angular position of the vibration tube 11.
In the conventional apparatus of the type described, the vibration tube 1 is fixed at its both ends. However, when the size of the flowmeter is limited, it is extremely difficult to perfectly fix both ends of the vibration tube so as to completely isolate the tube from vibration.
Two major problems are encountered with the conventional apparatus.
One of these problems is that the flowmeter is susceptible to external conditions.
More specifically, the housing of the flowmeter by itself cannot fully accommodate the influence of any vibration or stress of external piping, so that such external vibration or stress is transmitted to the internal vibration tube 11 to cause a change in the mode of vibration of the tube 11, resulting in fluctuation of the output and errors such as shifting of zero point.
The other problem is that the vibration of the internal vibration tube is propagated externally of the flowmeter.
External propagation of the vibration and insufficient isolation from external vibration cause the following drawbacks.
(1) Internal vibration is rendered unstable due to low Q value, enhancing susceptibility to vibration noise other than intentionally ocillated vibration.
(2) Electrical power consumption is increased due to large energy used for ocillatation.
(3) External propagation of vibration is significantly affected by external factors such as the manner of installation, stress in the piping and change in ambient conditions such as temperature, with the result that the mode of vibration of the vibration tube 11 is varied to allow easy change of the zero point and the span.
In the arrangement shown in FIG. 75, each point on the vibration tube 11 performs simple harmonic oscillation along an arc or a part of a circle of a predetermined radial distance from the reference axis 14 which is defined as being the straight line interconnecting the upstream end 12 and the downstream end 13 of the vibration tube 11.
The force acting on each fixed end of the vibration tube under the described ocillated vibration is mainly composed of torque or rotational component acting about the reference axis. This offers more effective isolation from vibration than in the arrangement shown in FIG. 74. However, the position of the center of gravity of the whole vibration system is shifted due to the change in the position of the vibration tube caused by the ocillated vibration.
Shift of the gravity center allows easy external propagation of vibration from the flowmeter, so that the problem in regard to the isolation from vibration still remains unsolved.
FIG. 29 is a plan view of a critical portion of a known Coriolis mass flowmeter of the type disclosed in Japanese Unexamined Patent Application Publication No. 61-189417. FIG. 30 is a side elevational view of the structure shown in FIG. 29. FIGS. 31 and 32 are illustrations of the operation of the known flowmeter shown in FIG. 29.
Referring to these Figures, a vibration tube has a first branch tube 218 and a second branch tube 219 which are in parallel with each other and which are supported by support plates 241 and 242 at their both ends.
A pair of vibration sensors 223 and 224 and an ocillator 221 are connected between these two branch tubes 218 and 219, so that these branch tubes are ocillated to constantly vibrate at their resonance frequency.
In most cases, the branch tubes perform ocillated vibrations in a basic resonance mode as illustrated in FIG. 31. More specifically, the first branch tube 218 vibrates to change its position from A to B, from B to A, from A to C and back again to A and then again to B and so on. In the meantime, the second branch tube 219 vibrates to change its position from Axe2x80x2 to Bxe2x80x2, from Bxe2x80x2 to Axe2x80x2, from Axe2x80x2 to Cxe2x80x2 and back again to Axe2x80x2 and then again to Bxe2x80x2 and so on. These two branch tubes vibrate in opposite phases in symmetry with each other.
It is also possible to arrange such that the branch tubes 218 and 219 vibrate in a high-order resonance mode as illustrated in FIG. 32. In this case also, the first branch tube 218 vibrates to change its position from A to B, from B to A, from A to C and back again to A and then again to B and so on, while the second branch tube 219 vibrates to change its position from Axe2x80x2 to Bxe2x80x2, from Bxe2x80x2 to Axe2x80x2, from Axe2x80x2 to Cxe2x80x2 and back again to Axe2x80x2 and then again to Bxe2x80x2 and so on. These two branch tubes vibrate in opposite phases in symmetry with each other. This vibration mode has a node of vibration on each branch tube and requires, for example, a pair of ocillators unlike the arrangements of FIG. 29 which uses a single ocillator.
This known Coriolis mass flowmeter employs vibration tubes 218 and 219 which are straight and which are fixed at their both ends as seen in FIG. 29. Therefore, when the positions of these branch tubes 218 and 219 have shifted from their neutral or unocillated positions A, Axe2x80x2 to positions B, Bxe2x80x2, the overall lengths of the branch tubes vary to produce large axial forces acting on the branch tubes 218, 219.
More specifically, as the branch tubes 218 and 219 are deformed due to the vibration in the manner shown in FIG. 31 or 32, axial forces are generated to pull the fixed ends of the branch tubes inward as indicated by arrows F1.
Such unnecessary forces acting on the fixed ends of the branch tubes causes the vibrations of the branch tubes 218 and 219 to be externally propagated, thus hampering isolation of vibration.
The inferior vibration isolation, i.e., a lower Q value, causes the following problems.
(1) Ocillatation requires large electric currents, leading to an increase in the electrical power consumption.
(2) External propagation of vibration largely varies depending on environmental conditions and external factors, with the result that the vibration is rendered unstable and unsteady, allowing output errors due to a shift of zero point, change in the span, and so forth.
The externally propagated vibration is reflected and introduced again into the internal vibration system. The reflected vibration has the same frequency as the internal vibration and, therefore, acts as a large noise even when the magnitude is small, and causes an error in the output.
The following problem is caused when the vibration tube is a single tube with no branching portion and having a curvilinear configuration devoid of any large curvature, as in FIG. 75.
The vibration tube is slightly expanded or contracted in the direction of the tube axis, i.e., in the direction of X-direction, when heat is applied in the course of welding or when there is a significant temperature difference between the vibration tube 11 and the housing 6. Such a slight axial deformation appears as a large deformation in the direction of curvature of the vibration tube, i.e., in the Z-direction, as shown in FIG. 46.
The amount of the deformation in the Z-direction depends on the configuration of the vibration tube 11 but is often several times greater than that of the amount of the axial deformation, well reaching several millimeters of greater, at the position where the ocillator is provided.
Such a large deformation tends to cause a magnet or a coil attached to the vibration tube to deviate away from the position of the associated coil or magnet which is fixed to a stationary part such as the housing 6.
This problem is serious particularly when the ocillator has a cylindrical coil and a magnet received in the coil, because the positional deviation may allow the coil and the magnet to mechanically interfere with each other, thus hampering stable vibratory operation and, in the worst case, causing breakage of the ocillator.
Accordingly, it is a primary object of the present invention is to provide a Coriolis mass flowmeter which is improved in stability, measuring accuracy and vibration proof performance, thereby overcoming the above-described problems.
Practically, the present invention provides a Coriolis mass flowmeter, wherein the vibration tube has a gently-curved curvilinear configuration approximating a straight configuration, thus implementing a compact structure which suffers reduced pressure loss and which is highly resistant to thermal stress caused by, for example, a change in the fluid temperature.
It is also an object of the present invention to provide a Coriolis mass flowmeter which excels in the isolation of the internal structure of the flowmeter from external vibration, thus suppressing influence of external vibration noise and reducing shifting of zero point, while achieving high measuring accuracy and stability.
It is also an object of the present invention to implement a Coriolis mass flowmeter which is improved to suppress both external propagation of vibration from the vibration tube and influence of external noise and stress, thus achieving high stability and measuring accuracy, as well as enhanced vibration-proof performance and reduced shift of zero point.
It is also an object of the present invention to provide a Coriolis mass flowmeter which has an ocillator capable of performing a stable ocillating operation without being influenced by thermal distortion of the vibration tube.
To these ends, according to one aspect of the present invention, there is provided a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube; wherein the vibration tube has a curved configuration which is point-symmetry with respect to the median point between an upstream fixed end and a downstream fixed end of the vibration tube and which has three inflection points, and performs, while maintaining the curved configuration, simple harmonic oscillation such that each point on the vibration tube oscillates on an arc of a predetermined radius from a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube.
Preferably, the Coriolis mass flowmeter further comprises: an upstream ocillator and a downstream ocillator which are provided on the vibration tube; an upstream compensation vibrator extending along the, reference axis, the upstream compensation vibrator having one end fixed to the upstream ocillator so as to receive from the upstream ocillator a torsional force which acts around the reference axis in the phase inverse to that of the torsional force acting on the vibration tube, the other end of the upstream compensation vibrator being fixed to a portion of the vibration tube near the upstream fixed end so that the torsional force of the inverse phase and the torsional force on the vibration tube cancel each other at the fixed other end of the upstream compensation vibrator; and a downstream compensation vibrator extending along the reference axis, the upstream compensation vibrator having one end fixed to the downstream ocillator so as to receive from the downstream ocillator a torsional force which acts around the reference axis in the phase inverse to that of the torsional force acting on the vibration tube, the other end of the downstream compensation vibrator being fixed to a portion of the vibration tube near the downstream fixed end so that the torsional force of the inverse phase and the torsional force on the vibration tube cancel each other at the fixed other end of the downstream compensation vibrator.
In accordance with a second aspect of the present invention, there is provided a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows; ocillators for ocillating the vibration tube to vibrate the vibration tube; and vibration sensors for sensing deformative vibration of the vibration tube caused by Coriolis force generated through cooperation between the flow of the fluid and angular vibration of the vibration tube; wherein the vibration tube has a curved configuration which is point-symmetry with respect to the median point between an upstream fixed end and a downstream fixed end of the vibration tube and which has three inflection points, and performs, while maintaining the curved configuration, simple harmonic oscillation such that each point on the vibration tube oscillates on an arc of a predetermined radius from a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube; and wherein the ocillators and the vibration sensors are arranged on the vibration tube such that the locations and masses of the ocillators and the vibration sensors are point symmetry with respect to the median point, and such that the center of gravity is located on the median point.
The second aspect of the present invention also provides a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows, the vibration tube having a curved configuration which is point-symmetry with respect to the median point between an upstream fixed end and a downstream fixed end of the vibration tube and which has three inflection points, the vibration tube performing, while maintaining the curved configuration, simple harmonic oscillation such that each point on the vibration tube oscillates on an arc of a predetermined radius from a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube; ocillators for ocillating the vibration tube to vibrate the vibration tube, each of the ocillators having a magnet attached to the vibration tube and a coil disposed to face the magnet; vibration sensors for sensing deformative vibration of the vibration tube caused by Coriolis force generated through cooperation between the flow of the fluid and angular vibration of the vibration-tube, each of the vibration sensors having a magnet attached to the vibration tube and a coil disposed to face the magnet; and balancers attached to the vibration tube so as to cancel mass un balance caused by vibration of the magnets; wherein the magnets and the balancers are arranged on the vibration tube such that the locations and masses of the magnets and the balancers are point symmetry with respect to the median point, and such that the center of gravity is located on the median point.
Preferably, the vibration tube is gently curved with small curvatures so as to be small in size and so as to reduce pressure loss of the fluid flowing therethrough.
It is also preferred that the distance W of a point on the vibration tube where the curvature is greatest from the reference axis substantially falls within the range of:
xc2x10.01xe2x89xa6W/Lxe2x89xa6xc2x10.1
where L indicates the overall length of the vibration tube.
In accordance with a third aspect of the present invention, there is provided a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube wherein the vibration tube includes: a branching portion where the flow of the fluid branches into first and second channels; a merging portion where the first and second channels merge in each other; a first branch tube defining the first channel, the first branch tube having a curved configuration with three inflection points and point symmetry with respect to a first median point which is midst between a first upstream fixed end near the branching portion and leading to the first channel and a first downstream fixed end near the merging portion and leading from the first channel; a second branch tube defining the second channel having one end connected to the branching portion and the other end connected to the merging portion, and having the same configuration as the first branch tube, the second branch tube extending in parallel with the first branch tube; the Coriolis mass flowmeter further comprising: ocillators fixed to the first and second branch tubes for ocillating the first and second branch tubes in such a manner that each point on the first branch tube performs simple harmonic oscillation on an arc of a predetermined radius from a first reference axis which is the straight line which interconnects the first upstream fixed end and the first downstream fixed end, while each point on the second branch tube performs simple harmonic oscillation on an arc of a predetermined radius from a second reference axis which is the straight line which interconnects a second upstream fixed end and a second downstream fixed end, and that the first and second branch tubes are in plane-symmetry with respect to a reference plane which is at an equal distance from a first plane containing the first branch tube and a second plane which is parallel to the first plane and contains the second branch tube; and vibration sensors fixed to the first and second branch tubes, for sensing vibration of the first and second branch tubes.
Preferably, lead lines from the ocillators or from the vibration sensors are led externally of the mass flowmeter through the median points, through the upstream fixed ends, or through the downstream fixed ends.
It is also preferred that the Coriolis type mass further comprises: a first balancer attached to the first branch tube and a second balancer tube attached to the second branch tube; wherein the first balancer is provided such that the center of gravity of a system including the first branch tube, the ocillator or the vibration sensor attached to the first branch tube and the first balancer coincides with the median point of the first branch tube, and the second balancer is provided such that the center of gravity of a system including the second branch tube, the ocillator or the vibration sensor attached to the second branch tube and the second balancer coincides with the median point of the second branch tube.
The present invention in its fourth aspect provides a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube; wherein the vibration tube has a curved configuration which is point-symmetry with respect to the median point between an upstream fixed end and a downstream fixed end of the vibration tube and which has three inflection points, and performs, while maintaining the curved configuration, simple harmonic oscillation such that each point on the vibration tube oscillates on an arc of a predetermined radius from a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube, the Coriolis mass flowmeter further comprising an ocillator provided on the median point and arranged to apply to the vibration tube a torque about the reference axis or the central axis of the vibration tube.
The fourth aspect of the present invention provides also a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube; comprising ocillators provided on portions of the vibration tube near an upstream fixed end and near a downstream fixed end of the vibration tube, so as to apply to the vibration force a torque about the central axis of the vibration tube or about a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube.
Preferably, the ocillators comprise at least one pair of ocillators, two ocillators of the pair being arranged along the outer peripheral surface of the vibration tube in symmetry with each other with respect to a plane containing the central axis of the vibration tube, the ocillators being secured to the vibration tube obliquely to the central axis so as to perform expansion and contraction obliquely to the central axis in opposite phases to each other, thereby applying torsional force to the vibration tube.
In accordance with the fifth aspect of the present invention, there is provided a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube; wherein the vibration tube has a curved configuration which is point-symmetry with respect to the median point between an upstream fixed end and a downstream fixed end of the vibration tube and which has three inflection points, and performs, while maintaining the curved configuration, simple harmonic oscillation such that each point on the vibration tube oscillates on an arc of a predetermined radius from a reference axis which is a straight line interconnecting the upstream fixed end and the downstream fixed end of the vibration tube; the Coriolis mass flowmeter further comprising: vibration sensors arranged to oppose each other across or around at a plane which is equally spaced from the upstream fixed end and the downstream fixed end of the vibration tube.
Preferably, the vibration sensors includes: a first vibration sensor disposed on a straight line which contains the median point and which is perpendicular to a vibration tube plane containing the curve of the vibration tube, the first vibration sensor being sensitive only to the vibration component that is perpendicular to the vibration tube plane; and a second vibration sensor disposed on a straight line which contains the median point and which is sensitive only to the vibration component that is parallel to the vibration tube plane and perpendicular to the reference axis.
Alternatively, the vibration sensors are arranged on a straight line which contains the median point and which is perpendicular to a vibration tube plane containing the curve of the vibration tube, such that the vibration sensors are highly sensitive to the vibration component in the direction perpendicular to the vibration tube plane and has small sensitivity to the vibration component in the direction parallel to the vibration tube plane and perpendicular to the reference axis, thereby increasing the ratio of the amplitude of Coriolis vibration to the amplitude of the ocillated vibration.
The arrangement also may be such that the vibration sensors are arranged in point-symmetry with respect to the median point and have sensitivity only to vibration components in the direction perpendicular to the vibration tube plane which contains the curve of the vibration tube, thereby increasing the ratio of the amplitude of Coriolis vibration to the amplitude of the ocillated vibration.
In accordance with a sixth aspect of the present invention, there is provided a Coriolis mass flowmeter, comprising: a vibration tube through which a fluid to be measured flows so that the flow of the fluid in cooperation with angular vibration of the vibration tube generates Coriolis force which causes vibratory deformation of the vibration tube; wherein the vibration tube includes: a branching portion where the flow of the fluid branches into first and second channels; a merging portion where the first and second channels merge in each other; a first branch tube defining the first channel, the first branch tube having a curved configuration with three inflection points and point symmetry with respect to a first median point which is midst between a first upstream fixed end near the branching portion and leading to the first channel and a first downstream fixed end near the merging portion and leading from the first channel; a second branch tube defining the second channel having one end connected to the branching portion and the other end connected to the merging portion, and having the same configuration as the first branch tube and contained in the same plane as the first branch tube, the position of the second branch tube being determined by translationally moving the position of the first branch tube; the Coriolis mass flowmeter further comprising: ocillators for ocillating the first and second branch tubes; and vibration sensors for sensing the vibration of the first and second branch tubes.
The ocillators may be arranged for ocillating the first and second branch tubes while maintaining the point-symmetry configurations of the first and second branch tubes, in such a manner that each point on the first branch tube performs simple harmonic oscillation on an arc of a predetermined radius from a first reference axis which is the straight line which interconnects the first upstream fixed end and the first downstream fixed end, while each point on the second branch tube performs simple harmonic oscillation on an arc of a predetermined radius from a second reference axis which is the straight line which interconnects a second upstream fixed end and a second downstream fixed end, and that the first and second branch tubes vibrate in the same vibration mode and at the same frequency in opposite phases to each other.
The arrangement may be such that lead lines from the ocillators or from the vibration sensors are led externally of the mass flowmeter through the median points, through the upstream fixed ends, or through the downstream fixed ends.
The Coriolis mass flowmeter of the sixth aspect may further comprise: a first balancer attached to the first branch tube and a second branch tube attached to the second branch tube; wherein the first balancer is provided such that the center of gravity of a system including the first branch tube, the ocillator or the vibration sensor attached to the first branch tube and the first balancer coincides with the median point of the first branch tube, and the second balancer is provided such that the center of gravity of a system including the second branch tube, the ocillator or the vibration sensor attached to the second branch tube and the second balancer coincides with the median point of the second branch tube.
These and other objects, features and advantages of the present invention will become clear from the following description of preferred embodiments taken in conjunction with the accompanying drawings.